The invention herein relates to filtration processes. More particularly it relates to processes for filtering biopolymers.
In tertiary oil recovery processes it is conventional to use polymeric materials in the flooding solution which is pumped through the oil-bearing formation to scavenge oil from the rock. Commonly the two types of polymers used are polyacrylamides and biopolymers. Biopolymers are preferred over the polyacrylamides because during the recovery process the polymer solution must be pumped at high pressure and high shear rates through the formation, and the polyacrylamides are much more shear sensitive than are the biopolymers. Biopolymers, however, have the disadvantage that they can cause wellbore impairment due to the presence of unhydrated biopolymer material and/or bacterial debris. Thus, in order for biopolymers to be successfully used in tertiary oil recovery they must be subjected to filtration to remove the unhydrated biopolymer material and the bacterial debris which is present.
Filtration of these components, particularly the bacterial debris, has in the past proved to be exceedingly difficult. The bacterial debris particles are quite small in size (usually on the order of 1 to 2 microns), are commonly coated with bipolymer, and have tendency to act as deformable solids. Further, since the viscosity of a biopolymer solution is higher than that of water the concentration of the filterable solution is limited.
It has been known in the past that subjecting biopolymer solutions to an alkaline environment would materially aid the filterability of the solution. Thus, washing of the polymer solution with an alkaline wash produces some improvement in the filterability of the polymer. However, cycle times (the period of time starting with the beginning of filtration and ending when the filter has become sufficiently clogged to raise the required filter pressure to an undesirable or otherwise predetermined level) are often still quite short and only small amounts of material can be filtered before the filter cycle ends. Further, elaborate processing equipment is required to meter the alkaline wash into the biopolymer solution, and maintenance of correct pH during filtration is difficult.
Diatomite filtration has been used with some degree of success in the past. However, neutral pH diatomites require the supplemental use of an alkaline component to obtain the correct pH. Alkaline diatomites are commonly too coarse to filter the small bacterial debris effectively. Similarly, adsorbent materials have been considered for biopolymer filtration, but these have the disadvantage that they remove desirable materials as well as undesirable materials.
It would thus be advantageous to have a biopolymer filtration process which utilizes solid filtration materials but which is selective to remove substantially only the undesirable components of the biopolymer solution, and which provides a correct and stable pH environment without the need for elaborate equipment and/or controls or use of supplemental materials.